1. Field of the Invention
The present invention relates to a variable focal length lens system and, in particular, relates to a variable focal length lens system having a compactness and a large zoom ratio suitable for an objective lens used for a lens shutter type camera.
2. Related Background Art
It has recently become common to use a zoom lens in a picture-taking optical system for a camera. In particular, since portability is regarded as important for a lens shutter type camera, In order to reduce size and weight of a camera itself, a small-sized lightweight zoom lens used for an objective lens is proposed.
As a zoom lens has become popular for an objective lens, various proposals have been made regarding a zoom lens having a long focal length in a telephoto end state and a large zoom ratio (a quotient of a focal length in the telephoto end state divided by a focal length in the wide-angle end state). A long focal length in the telephoto end state makes it possible to take picture closer to an object and a large zoom ratio makes it possible to take picture with high degree of freedom.
In these zoom lenses, because of the long focal lengths in the telephoto end state, the total lens lengths in the telephoto end state become long, so that the portability is spoiled. Therefore, the portability while being carried not for taking picture is improved by means of storing lens groups in the camera body in a state where the distances between adjacent lens groups are made minimum. By using such structure for the lens barrel that plurality of lens barrel elements are nested, it is possible to accommodate each lens barrel element as if the lens barrel element is folded when the camera is carried, so the portability while being carried is improved.
However, as the focal length in the telephoto end state becomes long, the barrel length also becomes long, so that each lens barrel element consisting of the lens barrel also becomes long. Then, the portability is spoiled in consequence. Accordingly, it is conceivable to reduce the thickness of the camera body while being accommodated by means of increasing the number of lens barrel element and, then, reducing the thickness of each lens barrel element. On the contrary in this case, however, the height and width of the camera become larger because the diameter of each barrel becomes larger, so the portability is also spoiled.
According to an optical design, it can be conceivable to avoid inconvenience of portability by means of shortening the total length of the lens in the telephoto end state. In a conventional zoom lens, in order to shorten the total length of the lens system in the telephoto end state, it is necessary to increase refractive power of each lens group or to increase the number of movable lens group. Because of the increase of the number of the lens element in each case, this is the result is contrary to objections of compactness and lightweight design and, therefore, contrary to the objective of good portability.